1. Technical Field
The present invention relates to an optical amplifier apparatus. More particularly, this invention relates to the optical amplifier apparatus which is utilized for an optical repeater and others for wavelength multiplex transmission.
2. Background Art
In recent years, along the rapid increase in demand for communications using the Internet and the like, there have been actively progressed researches and developments into and practical application of large-capacity and high-speed transmission systems in networks. As one of such signal transmission systems, a wavelength multiplex transmission system for transmitting signals by multiplexing signal lights of a plurality of mutually different wavelengths through one transmission fiber has been known. According to signal transmission based on the wavelength multiplex transmission system, it is possible to transmit information for each signal of each wavelength, and this system is suitable for large-capacity transmission. An optical fiber amplifier (an optical amplifier) constructed of a rare-earth-added optical fiber has the following characteristics, and is suitable for application to wavelength multiplex transmission. The amplifier does not depend on a transmission speed, the amplifier can simplify repeaters, and the amplifier collectively amplifies all input signal lights.
However, the optical amplifier has gain wavelength dependency, and it has been known that there occurs a variance between wavelengths in the light output or the gain of each wavelength after the amplification. Therefore, there occurs a variance between wavelengths in the optical power after the transmission. Particularly, when carrying out a multi-stage repeating by the optical amplifier, the variance between wavelengths due to the optical amplifier at each repeating stage is accumulated. This brings about the inconvenience that the variance between wavelengths of the optical power after the transmission becomes larger.
As an example of an optical amplifier gain equalization technique for solving the above problem, there is a technique (an optical equalization amplifier) as disclosed in Japanese Patent Application Laid-open No. 9-211507. This optical equalization amplifier is an apparatus for equalizing the output levels of a plurality of wavelength components as well as making constant the light input and output levels, in the amplification of wavelength-multiplexed lights. As shown in FIG. 17, the optical equalization amplifier has an optical amplifier 103 and a variable optical attenuator 104 arranged between an input terminal 101 and an output terminal 102.
The optical amplifier 103 has such gain inclination characteristics, that a gain at a long wavelength side becomes smaller when the excitation ratio is high, and that a gain at a short wavelength side becomes smaller when the excitation ratio is low. In other words, the optical amplifier 103 has a negative inclination of gain relative to wavelength when the excitation ratio is high, and has a positive inclination of gain relative to wavelength when the excitation ratio is low. As an example of a means which changes the excitation ratio, there are an excitation power control unit, and an optical level control unit for controlling a signal light that is input to the amplifier.
In the above optical amplifier apparatus, an input wavelength-multiplexed signal is amplified by the optical amplifier 103. A level of each wavelength is detected at the output side of the optical amplifier 103. The optical amplifier apparatus controls the excitation power (gain control) of the optical amplifier 103 so that the levels (gains) of the wavelengths are equalized. An output level is detected at the output side of the variable optical attenuator 104. The optical amplifier apparatus controls (output control) the attenuation of the output signal light of the variable optical attenuator 104.
However, a transmission line and a passive device have wavelength dependency attenuation, and therefore, optical signals of different levels are received at the input of the optical amplifier. This brings about a variance in the optical levels between the wavelengths. A variance of optical levels generated by one optical amplifier is small. However, as dozens to hundreds of optical amplifiers are provided for long-distance transmission like a submarine cable that connects between continents, the accumulation of such variances results in a small optical level in a channel of a specific signal wavelength. This has a risk of deteriorating an optical signal to noise ratio.
As the wavelength signal of the lowest power among the multiplexed wavelengths becomes a lower limit of reception power after the transmission, a maximum transmission distance is limited by the wavelength signal of the lowest power. Therefore, it is important to lower the variance between wavelengths after the transmission, in order to expand the maximum repetition transmission distance. For minimizing the variance in the optical signal to noise ratio between channels after the transmission and minimizing the variance in the reception characteristics, there is a technique of providing a gain inclination at the transmission side in advance (pre-emphasizing). An inline automatic gain inclination compensator may be inserted into the transmission line.
However, the provision of a large pre-emphasis lowers an optical SNR, and this also distorts a signal waveform due to a nonlinear effect of the transmission line attributable to the rise in signal optical power of a distance average. The pre-emphasis has inconvenience that the pre-emphasis effect becomes smaller as the transmission distance becomes longer. On the other hand, the inline automatic gain inclination compensator can minimize excessive deterioration of the optical SNR and the reception characteristics. However, there are other problems like the reliability of this compensator and the increase in the cost of the apparatus.